Residual stress in thin films made by atomic layer deposition

Residual stress in thin films bends wafers used as substrates. The curved wafer may cause problems in subsequent processing e.g. in lithography or measurements which assume planar wafers. Highly stressed films cause also reliability problems e.g. in sense of poor adhesion. In part, thin film stress is caused by thermal mismatch between the substrate and the film, while intrinsic thin film stress accounts for another part. Residual stress of thin films made by atomic layer deposition (ALD) has been occasionally reported, but so far, there have been little systematic studies on the stress of ALD films. We have in a large collaborative project grown and characterized by wafer curvature methods (Stoney equation) a wide series of thin films made by ALD, including Al₂O₃, TiO₂, SiO₂, TiN, AlN, NbN, and TiAlC_xN_y. Some results have been published earlier [1][2][3], while most have remained unpublished. Most films were made by "regular" ALD, while some employed energy-enhanced methods such as the use of ozone as a reactant or plasma enhanced ALD (PEALD). Most of the films were under tensile stress, while a few had close to zero stress, or were slightly on the negative (compressive) side (Fig. 1). All films that had zero or compressive stress, originated from growth where ozone or plasma was used as reactant, or reactant decomposition was likely to occur. The purpose of this presentation is to describe the limits in the wafer curvature thin film stress measurement; overview the stress results obtained for various ALD thin films and report trends observed in the thin film stress; and discuss means to control the stress of films made by ALD.